Fourier transform infrared (FTIR) spectrometers are utilized to provide accurate and efficient identification of the chemical composition of a sample. Such spectrometers typically incorporate a Michelson interferometer having a moving mirror. The interferometer modulates the infrared beam from an infrared source to provide an output beam in which the intensity of the infrared radiation at various wavelengths is periodically varied. The output beam is focused and passed through (or is reflected from) a sample, after which the beam is collected and focused onto a detector. The detector provides a time varying output signal which contains information concerning the wavelengths of infrared absorbence (or specular reflectance) of the sample. Fourier analysis is performed on the output signal data to yield usable information on the chemical composition of the sample.
With conventional FTIR instruments, the spectrometer is aligned during manufacture to provide optimum throughput based on published specifications. The overall height and geometry of the system optics generally define the height and lateral position of the focus, which is not normally specified, within the sample compartment at which the sample should be located. However, because of variations within the system optics the actual position of the focus can vary by a significant amount (in terms of tenths of millimeters) from the nominal position of the focus. This does not cause any significant problem for traditional large aperture transmission-based sampling accessories, such as cells or cuvettes. It does, however, cause a major problem for micro-sampling, and for the use of any specialized optical accessory.
Recently, the applications of modern FTIR instruments have been expanded by the use of special sampling accessories that involve precise beam imaging. The precision required by such accessories is at least an order of magnitude better than that provided by the beam imaging of a modern spectrometer. To accomodate modern accessories in modern instruments, accessory manufacturers provide a large degree of adjustment on many of the critical optical components within the accessory. This enables a given accessory to be custom aligned to specific instruments. For many modern accessories this is often a difficult and time-consuming exercise, and it is frequently too complex for an inexperienced operator. Once an accessory is removed from an instrument there is the danger that the precise alignment will be lost. Also, if the accessory is placed in another instrument of the same model a complete re-alignment of the accessory is usually required Without the customized alignment procedure, an accessory will either operate inefficiently, or in many cases will not operate at all. An ideal situation would be for an accessory manufacturer to provide pre-aligned accessories. However, the optical path variations of most current instruments is too great to make this practical, thus requiring a procedure of accessory alignment on these instruments.